Group III-V compound semiconductors containing nitrogen (hereinafter referred to as “nitride semiconductors”) have a band gap energy corresponding to the energy of light having a wavelength in the infrared region to the ultraviolet region, and are therefore useful as a material for a light-emitting device that emits light having a wavelength in the infrared region to the ultraviolet region or a material for a light-receiving device that receives light having a wavelength in the infrared region to the ultraviolet region.
The nitride semiconductors also have strongly bonded atoms that form the nitride semiconductors, a high breakdown voltage and a high saturation electron velocity, and are therefore also useful as a material for an electronic device such as a high-temperature-resistant, high-output and high-frequency transistor.
Furthermore, the nitride semiconductors hardly damage the environment and have been receiving attention as easy to handle material.
In a nitride semiconductor light-emitting device including such a nitride semiconductor, a quantum well structure is generally employed as a light-emitting layer. When a voltage is applied, electrons and holes flow into the light-emitting layer by diffusion and recombine in a well layer of the light-emitting layer, thus producing light.
There have been reports that the emission intensity of an InGaN-based LED (Light-Emitting Diode) is increased by insertion of a strained-layer superlattice (hereinafter referred to as “SLS”) structure made of InGaN/GaN immediately below multi-quantum wells (hereinafter referred to as “MQWs”). The details of this physical phenomenon are unclear, however. It is also unclear whether or not the emission intensity of an LED depends on the number of repeated cycles in the SLS structure.
It is known that defects in the form referred to as V-pits (V-shaped pits) are present in a nitride semiconductor structure. For example, PTD 1 (Japanese Patent Laying-Open No. 2005-277423) discloses a structure in which a “hexagonal pyramid cavity” is formed in a surface of an LED chip.
Since the V-pits are defects, it is generally believed that the characteristics of an LED will be improved if the generation of the V-pits is suppressed. On the other hand, NPD 1 (A. Hangleiter, F. Hitzel, C. Netzel, D. Fuhrmann, U. Rossow, G. Ade, and P. Hinze, “Suppression of Nonradiative Recombination by V-Shaped Pits in GaInN/GaN Quantum Wells Produces a Large Increase in the Light Emission Efficiency”, Physical Review Letters 95, 127402 (2005)) reports the action of V-pits in MQWs. According to this report, the presence of V-pits in MQWs leads to narrow quantum wells in the inclined faces of the V-pits, and thus increases an effective band gap due to the effect of an increased quantum level energy and the like. Consequently, electrons and holes in the quantum wells are prevented from reaching the inside of the V-pits, whereby nonradiative recombination in the MQWs is suppressed.